The present invention relates to a permanent magnet type electric motor including a stator having a plurality of field magnetic poles each having a composite permanent magnet composed of a rare earth metal containing permanent magnet and a ferrite magnet and an auxiliary pole and, particularly, to an improvement of the field magnetic poles.
A permanent magnet type electric motor having a cylindrical yoke on an inner peripheral surface of which field magnetic poles each composed of a ferrite magnet and a rare earth metal containing permanent magnet are arranged is known in, for example, Japanese Utility Model Application Laid-Open No. 18642/1985. In such motor, it is in general that an axial length of the ferrite magnet is equal to that of the rare earth metal containing magnet. Since the residual magnetic flux density Br of the ferrite magnet is 4500 gausses at most even when it is formed of a high Br material, magnetic flux from a surface of the rare earth metal containing magnet, when it is juxtaposed with the ferrite magnet, loops toward the ferrite magnet and a flux density distribution in a magnetic pole surface tends to become non-uniform. As a result, there may be the so-called cogging due to the nonuniformity of flux density in the magnetic pole surface and a reduction of effective flux due to the flux looping. Further, since the rare earth metal is expensive, it is highly desired to reduce an amount of such material as small as possible, which may contribute to a reduction of weight of a resultant motor.